Cellulose esters are used extensively in lacquer coatings because they are compatible with many resins and additives, they exhibit good gap-filling properties, they dry quickly, they can be sanded or rubbed soon after application, they exhibit low toxicity, and they form a very aesthetically-pleasing coating on a variety of substrates. Such coatings can be applied with reproducible results and can be repaired or even removed with the use of solvent. Such lack of solvent resistance is an advantage for lacquer handling since equipment used to apply the lacquer can be readily cleaned and the coating can be repaired easily if damaged. However, good solvent and stain resistance of the applied coating also is highly desirable, particularly if the coating is intended to be protective in nature. Moreover, cellulose ester lacquer coatings tend to be rather soft and are easily scratched.
Thermoset coatings have excellent solvent resistance and hardness properties, but they also have serious disadvantages. Most are thermally cured or crosslinked and their use is thus limited to substrates which are stable at the curing temperatures which can be as high as 230.degree. C. Moisture-cured systems have been used to overcome the use of high cure temperatures but these have prolonged cure times and have humidity requirements for such curing. The use of ultraviolet radiation to transform a photocrosslinkable thermoplastic coating into a thermoset coating thus represents a potentially desirable improvement. Curing can occur over a period of seconds to yield a hard, stain-resistant coating. In this manner, the advantages of both thermoplastic and thermoset polymers can be maintained.
It is also highly desirable to limit the amount of solvent used to apply such coatings due to environmental and work exposure issues. The use of water dispersible resins and resins dissolved in reactive solvents represent two potential solutions.
Attempts have been made to prepare cellulose-based resins that seek to provide such desired coatings and methods of application; however, such attempts have major deficiencies. For example, U.S. Pat. Nos. 4,112,182; 4,490,516; 3,749,592; and 4,147,603 do not provide crosslinkable resins having the desired level of solvent resistance and hardness, nor do any of these references teach the use of these resins in water dispersible or reactive solvent coatings.
Photopolymerizable cellulose esters described in U.S. Pat. No. 4,565,857 display a wide range of reactivities. For example, cellulose acetate propionate grafted with 2-isocyanatoethyl methacrylate per mole of anhydroglucose units has a short pot life in the presence of peroxides or a photoinitiator such as Ciba Geigy's IRGACURE.RTM. 651 Photocuring Agent (2,2-dimethoxy-2-phenylacetophenone) and can gel unexpectedly.
Other cellulose esters such as cellulose acetate propionate grafted with 0.9 moles of m-isopropyl-alpha, alpha-dimethylbenzyl isocyanate per mole of anhydro-glucose units completely fail to crosslink when exposed to UV radiation in the presence of a photoinitiator.
U.S. Pat. No. 4,839,230 describes grafted cellulose esters prepared by reacting a hydroxy-functional cellulose ester with an acrylic based compound and m-isopropenyl-.alpha.,.alpha.-dimethylbenzylisocyanate.
U.S. Pat. Nos. 4,565,857 and 4,839,230 also require the use of isocyanates for attachment of the ethylenically unsaturated moieties to the polymer backbone. This requires the manufacturing process to be moisture free, otherwise the isocyanate moiety reacts with water to form urea by-products which can cause deleterious effects by crystallizing out in the films made from the described resins.